1. Field Of The Invention
This invention relates to downhole tools for use in well bores and methods of drilling such apparatus out of well bores, and more particularly, to such tools having drillable components therein made of non-metallic materials, such as engineering grade plastics.
2. Description Of The Prior Art
In the drilling or reworking of oil wells, a great variety of downhole tools are used. For example, but not by way of limitation, it is often desirable to seal tubing or other pipe in the casing of the well, such as when it is desired to pump cement or other slurry down tubing and force the slurry out into a formation. It then becomes necessary to seal the tubing with respect to the well casing and to prevent the fluid pressure of the slurry from lifting the tubing out of the well. Packers and bridge plugs designed for these general purposes are well known in the art.
When it is desired to remove many of these downhole tools from a well bore, it is frequently simpler and less expensive to mill or drill them out rather than to implement a complex retrieving operation. In milling, a milling cutter is used to grind the packer or plug, for example, or at least the outer components thereof, out of the well bore. Milling is a relatively slow process, but it can be used on packers or bridge plugs having relatively hard components such as erosion-resistant hard steel. One such packer is disclosed in U.S. Pat. No. 4,151,875 to Sullaway, assigned to the assignee of the present invention and sold under the trademark EZ Disposal packer. Other downhole tools in addition to packers and bridge plugs may also be drilled out.
In drilling, a drill bit is used to cut and grind up the components of the downhole tool to remove it from the well bore. This is a much faster operation than milling, but requires the tool to be made out of materials which can be accommodated by the drill bit. Typically, soft and medium hardness cast iron are used on the pressure bearing components, along with some brass and aluminum items. Packers of this type include the Halliburton EZ Drill.RTM. and EZ Drill SV.RTM. squeeze packers.
The EZ Drill SV.RTM. squeeze packer, for example, includes a lock ring housing, upper slip wedge, lower slip wedge, and lower slip support made of soft cast iron. These components are mounted on a mandrel made of medium hardness cast iron. The EZ Drill.RTM. squeeze packer is similarly constructed. The Halliburton EZ Drill.RTM. bridge plug is also similar, except that it does not provide for fluid flow therethrough.
All of the above-mentioned packers are disclosed in Halliburton Services Sales and Service Catalog No. 43, pages 2561-2562, and the bridge plug is disclosed in the same catalog on pages 2556-2557.
The EZ Drill.RTM. packer and bridge plug and the EZ Drill SV.RTM. packer are designed for fast removal from the well bore by either rotary or cable tool drilling methods. Many of the components in these drillable packing devices are locked together to prevent their spinning while being drilled, and the harder slips are grooved so that they will be broken up in small pieces. Typically, standard "tri-cone" rotary drill bits are used which are rotated at speeds of about 75 to about 120 rpm. A load of about 5,000 to about 7,000 pounds of weight is applied to the bit for initial drilling and increased as necessary to drill out the remainder of the packer or bridge plug, depending upon its size. Drill collars may be used as required for weight and bit stabilization.
Such drillable devices have worked well and provide improved operating performance at relatively high temperatures and pressures. The packers and plug mentioned above are designed to withstand pressures of about 10,000 psi and temperatures of about 425.degree. F. after being set in the well bore. Such pressures and temperatures require the cast iron components previously discussed.
However, drilling out iron components requires certain techniques. Ideally, the operator employs variations in rotary speed and bit weight to help break up the metal parts and reestablish bit penetration should bit penetration cease while drilling. A phenomenon known as "bit tracking" can occur, wherein the drill bit stays on one path and no longer cuts into the downhole tool. When this happens, it is necessary to pick up the bit above the drilling surface and rapidly recontact the bit with the packer or plug and apply weight while continuing rotation. This aids in breaking up the established bit pattern and helps to reestablish bit penetration. If this procedure is used, there are rarely problems. However, operators may not apply these techniques or even recognize when bit tracking has occurred. The result is that drilling times are greatly increased because the bit merely wears against the surface of the downhole tool rather than cutting into it to break it up.
While cast iron components may be necessary for the high pressures and temperatures for which they are designed, it has been determined that many wells experience pressures less than 10,000 psi and temperatures less than 425.degree. F. This includes most wells cemented. In fact, in the majority of wells, the pressure is less than about 5,000 psi, and the temperature is less than about 250.degree. F. Thus, the heavy duty metal construction of the previous downhole tools, such as the packers and bridge plugs described above, is not necessary for many applications, and if cast iron components can be eliminated or minimized, the potential drilling problems resulting from bit tracking might be avoided as well.
The downhole tool of the present invention solves this problem by providing an apparatus wherein at least some of the components, including pressure bearing components, are made of non-metallic materials, such as engineering grade plastics. Such plastic components are much more easily drilled than cast iron, and new drilling methods may be employed which use alternative drill bits such as polycrystalline diamond compact bits, or the like, rather than standard tri-cone bits.